Optical glass having nD =1.63±(1.5×10-2), vD =59.5±1.0 with a high chemical stability and a low tendency to crystallization

ABSTRACT

An optical glass having n D  =1.63±(1.5×10 -2 ), v D  =59.5±1.0, a high chemical stability and a low tendency to crystallization is provided, consisting essentially of (in percent. by weight): 
     
         ______________________________________                                    
 
    
     SiO 2                23-27                                             
B 2  O 3          24-30                                             
SrO                     18-31                                             
La 2  O 3         10-16                                             
Al 2  O 3         2-4                                               
BaO                     0-10                                              
CaO                     0-5                                               
ZnO                     0-6                                               
Y 2  O 3          0-5                                               
ZrO 2                0-2                                               
Li 2  O, Na 2  O and/or K 2  O                                   
                        0-1.5                                             
As 2  O 3  and/or Sb 2  O 3                                   
                        0-1                                               
F and/or Cl             0-1,                                              
______________________________________                                    
 
     wherein the sum of SiO 2  +B 2  O 3  is 50-53, the weight ratio SiO 2  :B 2  O 3  is 0.8-1.0, the sum SrO+BaO is 26-31, and the sum ZnO+La 2  O 3  is 12-17.

BACKGROUND OF THE INVENTION

The present invention relates to a glass having the optical characteristics n_(D) =1.630±(1.5·10⁻²) and v_(D) =59.5±1.0. Glasses having these optical characteristics are known (see for instance DE-PS No. 14 21 877=U.S. Pat. No. 3,248,238), whose disclosures are incorporated by reference herein). However, they all contain a high percentage of BaO and consequently have very little chemical stability.

SUMMARY OF THE INVENTION

Accordingly, it is an object of this invention to provide a glass having the above mentioned optical characteristics, a good chemical stability and, at the same time, such little tendency to crystallization that it can be produced in a crucible melting batch process or in a continuous tank melting process in the form of drops, pressings, billets or glass blocks of any desired shape.

Upon further study of the specification and appended claims, further objects and advantages of this invention will become apparent to those skilled in the art.

These objects have been attained by providing an optical glass having the optical characteristics n_(D) =1.63±(1.5×10⁻²), v_(D) =59.5±1.0, a high chemical stability and a low tendency to crystallization, and having the following composition (in percent. by weight):

    ______________________________________                                                     p     Preferred Range                                              ______________________________________                                         SiO.sub.2     23-27   23-27                                                    B.sub.2 O.sub.3                                                                              24-30   24-30                                                    SrO           18-31   18-22                                                    La.sub.2 O.sub.3                                                                             10-16   10-16                                                    Al.sub.2 O.sub.3                                                                             2-4     2-4                                                      BaO            0-10     7-9,5                                                  CaO           0-5     0-5                                                      ZnO           0-6     0-6                                                      Y.sub.2 O.sub.3                                                                              0-5     0                                                        ZrO.sub.2     0-2       0,5-1,5                                                Li.sub.2 O,Na.sub.2 O                                                          and/or K.sub.2 O                                                                               0-1.5   0-1,5                                                  As.sub.2 O.sub.3 and/or                                                        Sb.sub.2 O.sub.3                                                                             0-1     0-1                                                      F and/or Cl   0-1     0,2-0,8                                                  wherein the sum of the vitrifiers                                                 SiO.sub.2 + B.sub.2 O.sub.3 is 50-53,                                       the weight ratio                                                                  SiO.sub.2 : B.sub.2 O.sub.3 is 0.8-1.0 (molar ratio 1.0-1.1),               the sum                                                                           SrO + BaO is 26-31,                                                         and the sum                                                                       ZnO + La.sub.2 O.sub.3 is 12-17.                                            ______________________________________                                    

DETAILED DISCUSSION

Fully conventional glassmaking procedures can be employed to prepare the glasses of this invention. Advantageously, the mentioned crucible melting batch process (for details, see, e.g. U.S. Pat. No. 3,898,093, whose disclosure is incorporated by reference herein) or the continuous tank melting process (for details, see e.g., U.S. Pat. No. 3,970,466, whose disclosure is incorporated by reference herein), is used. In this regard, reference is made to the following examples.

The glasses of this invention are particularly well adapted for conventional use in objectives for cameras, projectors and similar instruments.

The chemical stabilities, and tendencies to crystallization as measured via the standard methods, disclosed in the examples below, of the glasses of this invention, generally have values of:

    ______________________________________                                                             range                                                      ______________________________________                                               chemical stability                                                       (a)   acid class, class f better than 5c                                       (b)   erosion of 0.1μ in min. in                                                                      more than 6 min.                                           standard acetate, pH 4.5                                                 (c)   stain test class F1 3-4                                                  (d)   thickness of layer                                                             destroyed after 1 hour                                                         in a solution, pH 5.6                                                                              <0.24μ                                            (e)   climatic change     better than class 4                                        stability after 30 hrs                                                   (f)   stray light         ≦4.5%                                               crystallization tendency                                                 (g)   no devitrification in the glass                                          (h)   crystal growth rate of                                                                             ≦1.2 μ/min.                                ______________________________________                                    

Of particular importance is the attainment of the combined properties (a) through (h). It is in this sense that these properties are described as high stability and low crystallization tendency.

Without further elaboration, it is believed that one skilled in the art can, using the preceding description, utilize the present invention to its fullest extent. The following preferred specific embodiments are, therefore, to be construed as merely illustrative, and not limitative of the remainder of the disclosure in any way whatsoever. In the following examples, all temperatures are set forth uncorrected in degrees Celsius; unless otherwise indicated, all parts and percentages are by weight.

EXAMPLE I

Certain embodiments of this invention as well as three known high-barium-content glasses A, B, C are specified by way of example in the accompanying table. Indicative of the actual weight quantities of batch ingredients for each embodiment, are those given below for Example 8 of the table.

Melt Example (Example 8, see table)

    ______________________________________                                                           weighed quantity for                                         Oxides            120 kg of glass, in kg                                       ______________________________________                                         SiO.sub.2                                                                              24.16         Sipur     29.035                                         B.sub.2 O.sub.3                                                                        27.49         H.sub.3 BO.sub.3                                                                         58.386                                         BaO     9.06          Ba(NO.sub.3).sub.2                                                                       18.536                                         Na.sub.2 O                                                                             1.12          NaF       1.833                                          Al.sub.2 O.sub.3                                                                       2.62          AlO(OH)   4.220                                          La.sub.2 O.sub.3                                                                       14.90         La.sub.2 O.sub.3                                                                         17.933                                         ZrO.sub.2                                                                              1.00          ZrO.sub.2 1.204                                          SrO     19.24         Sr(NO.sub.3).sub.2                                                                       47.602                                         F       0.68                                                                   ______________________________________                                    

    __________________________________________________________________________     Oxide   A    B    C    1    2    3    4    5    6    7    8                    __________________________________________________________________________     SiO.sub.2                                                                              32.50                                                                               29.45                                                                               26.42                                                                               23.65                                                                               24.00                                                                               25.12                                                                               24.00                                                                               24.12                                                                               24.28                                                                               25.90                                                                               24.16                B.sub.2 O.sub.3                                                                        16.40                                                                               19.30                                                                               23.27                                                                               26.92                                                                               27.30                                                                               27.45                                                                               27.30                                                                               27.45                                                                               27.55                                                                               24.30                                                                               27.49                Li.sub.2 O                                                                             --   --   --   --   --   --   --   --   --   --   --                   Na.sub.2 O                                                                             0.30 --   0.15 --   --   --   --   --   --   --   1.12                 K.sub.2 O                                                                              --   --   0.01 --   --   --   --   --   --   --   --                   MgO     --   --   --   --   --   --   --   --   --   --   --                   CaO     --   --   --   3.85 --   2.92 --   3.92 --   --   --                   SrO     --   --   0.45 18.41                                                                               19.10                                                                               18.77                                                                               19.10                                                                               18.77                                                                               21.71                                                                               30.91                                                                               19.24                BaO     49.00                                                                               48.35                                                                               44.93                                                                               7.74 9.00 7.89 9.00 7.89 7.93 --   9.06                 ZnO     --   0.40 3.44 4.81 2.00 3.94 1.00 2.94 --   --   --                   Al.sub.2 O.sub.3                                                                       1.30 1.80 0.52 2.50 2.60 2.55 3.50 2.55 2.58 3.30 2.62                 La.sub.2 O.sub.3                                                                       --   0.40 --   10.96                                                                               14.80                                                                               10.18                                                                               14.80                                                                               10.18                                                                               14.66                                                                               14.88                                                                               14.90                ZrO.sub.2                                                                              --   0.50 --   0.96 1.00 0.98 1.00 0.98 0.99 --   1.00                 WO.sub.3                                                                               --   0.10 --   --   --   --   --   --   --   --   --                   As.sub.2 O.sub.3                                                                       0.60 0.15 0.40 0.20 0.20 0.20 0.20 0.20 --   0.10 --                   AlF.sub.3                                                                              --   0.05 --   --   --   --   --   --   --   --   --                   (Cl')F.sub.2 --O                                                                       --   --   --   --   --   --   0.10 --   0.30 0.59 0.39                 F       --   --   '    --   --   --   0.24 --   0.51 1.02 0.68                 nd      1.6204                                                                              1.6204                                                                              1.6194                                                                              1.62749                                                                             1.6266                                                                              1.6212                                                                              1.6206                                                                              1.6252                                                                              1.6238                                                                              1.6210                                                                              1.6200               vd      60.33                                                                               60.33                                                                               60.22                                                                               58.50                                                                               59.00                                                                               59.10                                                                               59.10                                                                               59.50                                                                               59.90                                                                               60.00                                                                               60.00                s       3.619                                                                               3.581                                                                               3.563                                                                               3.351                                                                               3.382                                                                               3.298                                                                               3.357                                                                               3.328                                                                               3.377                                                                               3.390                                                                               3.363                Acid Resistance                                                                        5c   5b/c 5c   5b/c 5b   5b   5b   5b   5b   5b   5b                   class f                                                                        Erosion in mins                                                                        4.1  5.4  1.5  5.2  9.8  7    10.8 6    9.8  7.3  8.9                  up to 0.1μ                                                                  Stain class F1                                                                         5    4    4    4    4    4    3    3    4    4    3                    Layer thickness                                                                in μ destroyed                                                              after 1 hr.                                                                    in Soln. I pH                                                                          12'/0.18                                                                            --   --   --   --   --   --   --   --   --   --                   4.6                                                                            in Soln. II                                                                    pH 5.6  --   0.26 0.24 0.17 0.17 0.18 0.16 0.16 0.17 0.24 0.18                 Climatic change                                                                stability after                                                                        4    3    4    3    3    3    3    3    3    3    3                    30 hrs.                                                                        Stray Light                                                                            8.6  4.6  9.5  3.4  3.6  3.0  3.6  3.4  3.4  4.5  3.9                  __________________________________________________________________________

Each of the well mixed batches is introduced at 1300°-1350° C. into a 50 liter Pt-crucible, then heated up to 1370° C. and refined for approximately 5 hrs until bubble-free. The melt is then left to stand at 1300° C. and, at approximately 1150° C., run through a discharge pipe into pre-heated metal molds of the desired form to make billets. Alternatively, the melt is pressed to make drops or pressings. Using a suitable Zac-stone tank it is also possible to produce a melt of the above specified compositions continuously. This can also be processed into billets, drops and pressings as required.

EXAMPLE II (see Example 5 of the table)

The following describes by way of example an analysis of a glass according to this invention.

    ______________________________________                                         Composition (in weight percent.):                                              ______________________________________                                                SiO.sub.2                                                                            24.12                                                                    B.sub.2 O.sub.3                                                                      27.45                                                                    CaO   3.92                                                                     SrO   18.77                                                                    BaO   7.89                                                                     ZnO   2.94                                                                     La.sub.2 O.sub.3                                                                     11.18                                                                    Al.sub.2 O.sub.3                                                                     2.55                                                                     ZrO.sub.2                                                                            0.98                                                                     As.sub.2 O.sub.3                                                                     0.20                                                                     n.sub.D                                                                              1.6252                                                                   v.sub.D                                                                              59.5                                                              ______________________________________                                    

This glass has the following properties:

    ______________________________________                                         coefficient of expansion α 10.sup.7 /°C.                          between 20-300° C.                                                                           71                                                        transformation temperature                                                     Tg (°C.)      642                                                       fusing temperature EW (°C.)                                             at 10.sup.7.65 d Pa.s                                                                               739                                                       specific gravity s   3.328                                                     acid class class f   5b                                                        erosion of 0.1μ in min. in                                                  standard acetate, pH 4.5                                                                            6                                                         stain test class F1  4                                                         thickness of layer destroyed                                                   after 1 hour in a solution,                                                    pH 5.6               0.16μ                                                  climatic change stability                                                                           3.                                                        ______________________________________                                    

The chemical stability (tested by standard acid resistance-staining and climatic change stability tests as described, for example, in Schott pocket catalogue No. 3050, (1972)) is one class better than that of currently available conventional glasses having these optical characteristics.

The crystallization test (O. H. Grauer and E. H. Hamilton: Improved Apparatus for Determination of Liquidus Temperatures and Rates of Crystal Growth in Glasses; J. Research Natl. Bur. Standards, 44 (5), 495-502 (1950), RP 2096), after tempering for 60 minutes in a gradient furnace, shows no devitrification in the glass, and on the surface, minor surface crystals at 915° C. (corresponding to 1.4×10³ d Pa.s), with a crystal growth rate of 0.1 μ/min.

EXAMPLE III (see Example 8 of the table)

    ______________________________________                                         Composition in weight percent.                                                 ______________________________________                                         SiO.sub.2        24.16                                                         B.sub.2 O.sub.3  27.49                                                         Na.sub.2 O       1.12                                                          SrO              19.24                                                         BaO              9.06                                                          Al.sub.2 O.sub.3 2.62                                                          La.sub.2 O.sub.3 14.90                                                         ZrO.sub.2        1.00                                                          F                0.68                                                          n.sub.D          1.6200                                                        v.sub.D          60.02                                                         α.10.sup.7 /°C.                                                   (20-300° C.)                                                                             71                                                            Tg (°C.)  628                                                           EW (°C.)  730                                                           s                3.363                                                         f class          5b                                                            erosion of 0.1μ                                                             in min. in                                                                     standard acetate,                                                              pH 4.6           8.9                                                           F1 Class         3                                                             Layer thickness                                                                in μ destroyed                                                              after 1 hr in                                                                  solution at                                                                    pH 4.6           0.18                                                          Climatic change                                                                stability        3                                                             ______________________________________                                    

The crystallization test after 60 minutes showed no devitrification in the glass, minor devitrification at the surface at 970° C. (corresponding to 250 dPa.s), and a crystal growth rate of 1.2 μ/min.

The preceding examples can be repeated with similar success by substituting the generically or specifically described reactants and/or operating conditions of this invention for those used in the preceding examples.

From the foregoing description, one skilled in the art can easily ascertain the essential characteristics of this invention, and without departing from the spirit and scope thereof, can make various changes and modifications of the invention to adapt it to various usages and conditions. 

What is claimed is:
 1. An optical glass having n_(D) =1.63±(1.5×10⁻²), v_(D) =59.5±1.0, a high chemical stability and a low tendency to crystallization, and consisting of (in percent. by weight):

    ______________________________________                                         SiO.sub.2               23-27                                                  B.sub.2 O.sub.3         24-30                                                  SrO                     18-31                                                  La.sub.2 O.sub.3        10-16                                                  Al.sub.2 O.sub.3        2-4                                                    BaO                     0-10                                                   CaO                     0-5                                                    ZnO                     0-6                                                    Y.sub.2 O.sub.3         0-5                                                    ZrO.sub.2               0-2                                                    Li.sub.2 O, Na.sub.2 O and/or K.sub.2 O                                                                0-1.5                                                  As.sub.2 O.sub.3 and/or Sb.sub.2 O.sub.3                                                               0-1                                                    F and/or Cl             0-1,                                                   wherein the sum of                                                             SiO.sub.2 + B.sub.2 O.sub.3 is                                                                         50-53,                                                 the weight ratio                                                               SiO.sub.2 :B.sub.2 O.sub.3 is                                                                          0.8-1.0,                                               the sum                                                                        SrO + BaO is            26-31,                                                 and the sum                                                                    ZnO + La.sub.2 O.sub.3 is                                                                              12-17.                                                 ______________________________________                                    


2. An optical glass of claim 1 having the following properties:

    ______________________________________                                                             range                                                      ______________________________________                                               chemical stability                                                       (a)   acid class, class f better than 5c                                       (b)   erosion of 0.1μ in min. in                                                                      more than 6 min.                                           standard acetate, pH 4.5                                                 (c)   stain test class F1 3-4                                                  (d)   thickness of layer  <0.24                                                      destroyed after 1 hour                                                         in a solution, pH 5.6                                                    (e)   climatic change     better than                                                stability after 30 hrs                                                                             class 4                                              (f)   stray light         ≦4.5%                                               crystallization tendency                                                 (g)   essentially no devitrification                                           (h)   crystal growth rate of                                                                             ≦1.2 μ/min.                                ______________________________________                                    


3. An optical glass of claim 1 having the following property:acid class, class f: better than 5c.
 4. An optical glass of claim 1 having the following property:erosion of 0.1μ m in min. in standard acetate, pH 4.5.: more than 6 min.
 5. An optical glass of claim 1 having the following property:stain test class Fl: 3-4.
 6. An optical glass of claim 1 having the following property:thickness of layer destroyed after 1 hour in a solution, pH 5.6: <0.24.
 7. An optical glass of claim 1 having the following property:climatic change stability after 30 hours: better than class
 4. 8. An optical glass of claim 1 having the following property:stray light: ≦4.5%.
 9. An optical glass of claim 1 having essentially no devitrification.
 10. An optical glass of claim 1 having a crystal growth rate of ≦1.2 μ/min. 